The present invention relates to processing an image signal utilizing pulse width position modulation techniques in combination with multiple halftone processes to improve that image for reproduction.
Many printing devices are not capable of reproducing gray scale images because they are bi-level. As a consequence, binary representation of gray scale images is a requisite in a wide range of applications such as laser printers, facsimile machines, lithography (newspaper printing), liquid crystal displays and plasma panels. Gray scale images are typically converted to binary images utilizing halftone techniques. Halftoning renders the illusion of various shades of gray by using only two levels, black and white, and can be implemented either digitally (facsimile machines, laser printers) or optically (newspaper printing).
Most information display devices are binary in nature, whereas most images are continuous in tone. Therefore, the ability to display continuous tone images on binary devices is very useful. However, the problem of optimally displaying continuous images in a binary form remains unsettled. This problem arises in many forms of media transfer, from graphic arts to facsimile machines. Virtually all printed images in books, magazines, newspapers, etc. are composed in a binary nature. Computer hard copy devices are almost exclusively binary in nature. The process of transforming continuous gray scale information into binary information which is perceived to contain a continuous tone, is called halftoning.
A continuous image is one that can be defined as "natural". It is one which contains indistinguishable transitions from one gray level to the next. A binary image is one that is composed of picture elements that are either black or white. Therefore, the display of a gray scale image on a binary output device requires that the continuous image be quantized into two levels.
Desirable halftone algorithm characteristics include: implementation simplicity; reproduction accuracy for both low frequency (or constant) and high frequency (or edges in fine detail); and the absence of visual artifacts such as low frequency Moire pattern (aliasing) and false quantization contours (artificial boundaries). Essentially, the desired result of the halftoning process is such that the halftone images observed at normal viewing distances of 30-45 centimeters show dot dispersion which is perceived as varying gray levels, while the underlying dot structure remains unnoticed.
Ordered dither is a halftone technique which represents continuous tones with clusters of dots arranged to give darker or lighter patterns. Input values are compared against a fixed sized screen, and dots are added to the dither lattice with increasing gray levels. Ordered dither techniques include white noise, cluster-dot and dispersed-dot. The disadvantages of ordered dither algorithms include loss of most fine detail and the introduction of periodic artifacts. See Digital Halftoning by R. Ulichney, MIT Press, Cambridge, Mass. (1987).
The major ordered dither techniques are the clustered-dot dither and dispersed-dot dither techniques. Stochastic halftoning processes are possible but will be addressed later. Of the two techniques, clustered dot is by far the most used, since it reproduces well with xerographic and similar electrostatically based printing technologies. Both of those techniques are based upon a threshold array pattern that is of a fixed size. For example, 6.times.6 threshold arrays may be compared with the digital input values. If the input digital value is greater than the array pattern number, a 1 is produced and, if it is less, a 0 value is assigned. The number of levels that can be represented using either technique depends on the size of the array. For example, a 6.times.6 array can produce 36 unique levels. However, the larger the array the lower the scan frequency, and hence the greater the loss in picture detail.
When assessing the quality of a binary xerographic printer, two measures are important: the halftone frequency (i.e. number of halftone cells per linear inch), and the number of distinguishable gray steps. To produce a copy of a picture with a just acceptable degree of halftone graininess requires at least 65 halftone cells per inch measured along a diagonal of the page. Good quality halftones require about 100 cells/inch, while high quality magazines typically use 150 cells/inch or higher. The needed number of distinct gray steps in the pictorial copy depends upon the eye's ability to distinguish closely spaced grays. A rule of thumb in the printing industry is that an acceptable picture should contain about 65 gray steps. For good quality, 100 or more steps are desired. However, in a binary printer, the maximum number of output gray steps is limited to the number of pixels per halftone cell (p), plus 1. Thus for a typical 8 by 4 rectangular halftone cell, p+1=33 output gray steps. Higher halftone frequencies have fewer pixels per cell and therefore produce fewer gray steps. This is the fundamental limitation of binary printers.
More levels can be achieved with larger patterns, however, a reduction in the effective resolution occurs because the ability to transition among levels is at a coarser pitch. At the pixel rate of about 300 to 600 per inch, which is the average pixel rate of copiers and laser printers, the pattern artifacts are visible for screen patterns larger than 4.times.4, and, since 16 levels do not provide an adequate precision for typical continuous-tone imagery, a suboptimal resolution is usually obtained.
Line screening is another halftoning technique. Utilizing a raster output scanning (ROS) approach in combination with pulse width modulation (PWM) techniques, line screens enjoy good detail resolution and freedom from moire problems. This is particularly so when extended into high addressability (HA) by use of pulse width position modulation techniques (PWPM). High addressability is characteristic of a system with sub-pixel addressability. This is achieved with PWPM by using a laser with a spot size significantly smaller than a pixel, and by using positioning circuitry capable of starting and stopping the laser pulse at sub-increments of a pixel. This further increases the detail resolution of the system.
However, there are still difficulties with line screens and PWPM, in rendering faithful or pleasing copies of continuous tone originals. The usual discharge characteristic of the photoconductor and solid area developability of the xerographic development system combine to yield a Tone Reproduction Curve (TRC) with a steep slope and a narrow range. At one end of the gray scale spectrum there is a finite limit in the smallest amount of charge that can be developed and attendant limitations in the minimum amount of toner which can be expected to adhere to that charge. At the other end of the gray scale spectrum there is a point at which the volume of toner developed swamps out the small undeveloped areas. The result is a copy with washed out highlights and overdeveloped shadows.
One technique applied to this problem is taught in U.S. Pat. No. 5,587,772 to Arai et al. Disclosed here is a pulse width modulation system where the clock frequency for a line type halftoner is toggled between two frequencies, utilizing lower frequency line types to overcome cost and provide improved natural image quality without sacrificing character image quality. A discrimination device determines whether an image density signal belongs to a line image or to a natural image portion of an image. The discrimination device provides a signal to select from two image density conversion devices. The first image density conversion device having a image conversion property for application to most of the image density range except the low density portion. The second image density conversion device having a second conversion property different from the first and for converting a range of image density signal corresponding to a low density portion. A pulse width modulation device modulates the conversion signals output from the first and second image density conversion devices and provides a pulse width modulation signal. A pulse width modulation signal period changing device is provided for changing the period of the pulse width modulation device output from the pulse width modulator and the period of selection in accordance with the discrimination signal output from the image density signal discrimination device. The main purpose of the apparatus is to realize line-reduction owing to the property of one of the conversion means that the conversion means outputs a value within a range corresponding to portions where the electrostatic latent image is not developed.
Stochastic screen is yet another halftoning technique. A stochastic halftone cell is a large threshold array that produces random appearing patterns in the halftone image. One of the advantages of stochastic, or non-periodic screening over periodic screening is the suppression of moire. However it has a less desirable image quality, having higher image noise which leads to more grainy looking images than for example, clustered dot. U.S. Pat. No. 5,673,121 discloses an idealized stochastic screen, characterized by all of the predominant color dots (black or white) being uniformly distributed. The invention seeks to approach this optimization by iteratively selecting pairs of threshold levels in the screen matrix, and measuring the approach to the idealized stochastic screen. The threshold values are then swapped in position to determine whether the swap improves the measurement or not. If it does, the swap is maintained. The process is iterated until the desired result is obtained. It should be noted that stochastic screens are very desirable in implementing digital watermarks as taught in U.S. Pat. No. 5,734,752 to Knox, and incorporated by reference herein.
In U.S. Pat. No. 5,394,252 to Holladay et al., discloses an image processing system for preparing a color document for printing. Where each discrete area or pixel in the image is described by a signal having a number of possible states or color separations. Each separation of the image is halftoned, with at least one of the separations processed with a non-periodic halftoning method, and at least one of the remaining separations processed with a periodic pattern. Preferably, this is performed in a printer printing with colorants approximating cyan, magenta, yellow and black, where one of the non-yellow separations is processed with the non-periodic halftoning method. The teaching here is the avoidance of moire and color degradation due to spatial shifting by applying different halftone techniques to different separations. Advantage is made of utilizing different halftone techniques upon different separations where one image separation is printed over top another. However, no solution is provided to address the need for applying different halftone types to different segments or density areas within a single separation or image.
As may be noted from the discussion above, while there are many halftone techniques, they all posses strengths and weaknesses. Therefore, it would be desirable to have a method and apparatus which would allow the combination of different given halftone techniques with their attendant strengths to be applied to correspondingly appropriate segments within an image.